224 ORGANIC REMAINS. 



of the spines, of which it bears but equivocal traces, makes us doubt 

 whether it can be reckoned of this species or not. It may, however, 

 be a variety of it. 



As the waved ribs or striae appear to form a characteristic mark 

 of the species first mentioned, it may be proper to name it unclata, 

 rather than rigida; as the latter character does not appear essential. 

 It appears to be further distinguished by a complicated tooth-like 

 process at the end of the anterior ear, on that valve which is the least 

 cleft. This process being usually dressed away, by those who 

 discover or clear out the specimens, we have met with it only in two 

 or three instances. It is formed by a raised fold of part of the ear, 

 and a piece pi'ojecting from the end of that fold; but we are not 

 sure that we have seen it in a complete state. Mr. Parkinson fovmd 

 it in only one specimen, and considei-ed it as a lateral tooth : Mr. 

 Sowerby does not appear to have seen it at all; his observations being 

 chiefly made on p. spinosa, in which it is not found. In one of our 

 smoother specimens, this process, or something similar to it, appears 

 at the very beaks. Perhaps on a more full examination of the shells 

 of this singular genus, the form or disposal of this process may be 

 found to constitute an important specific character.* 



We may only add, that all the specimens of plagiostoma which 

 we have seen are from the oolite, or the grey limestone under it. 

 They are principally obtained from the neighbourhood of Malton, 

 particularly from Swinton. Several of them are much compressed. 



Trigonia. This new genus of Bruguiere, which has been dis- 

 covered recent in some parts of the world, may be introduced here, 



* See the figures referred to in this article, in Sowerby's Min. Concli. Tab. 77, 78, 113, 

 114. Parkinson's Org. Rem. III. PI. XIII. Fig. 6. Sowerby (p. 175) quotes the latter 

 erroneously as Fig. 3. The Scarborough Catalogue gives a good specimen in PI. I. Fig. 3 ; 

 but in the printed description, p. 107, the reference is made by mistake to Plate II. Fig. 1 j 

 ■which last is an indifferent specimen of the fossil which we have given in PI. VII. Fig. 12. 



